Question

why heating element of an electric kettle is at the bottom

Answer 1

because the flame of the gas fire is in direct contact only with the bottom of the utensil.
Hence, the heating element of not only the electric kettle but of all the utensils either electric or not is at the bottom so that the part receiving the maximum amount of heat will provide heat to the rest of the part more efficiently

Answer 2

"when an element us at the bottom ,one can boil any amount of water that will cover the element .hot water is less dense cold water ,so hot water rises from a hot element to the top of the water , giving off heat all the way , so the heating is uniform . this is called natural convection" .